This invention relates generally to an electrochemical cell for use in a gas detector unit. More particularly, this invention relates to a wick system useful in the functioning of a cell for detecting monomethylhydrazine, in that it provides for continuous and enhanced contact between the electrodes of the cell. The wick is comprised of a wettable material which absorbs electrolyte, and by capillary action maintains the continuous contact. Thus, as the gas detector unit is used over a period of time and the quantity of electrolyte diminishes, the cell will still operate because the wick continues to absorb and carry the electrolyte to the electrodes and maintains a continuous path between them. Moreover, the wick of the invention utilizes the internal vapor pressure of the electrolyte and will compensate for any water loss in those instances where an aqueous electrolyte system is being utilized.
This invention is an improvement over the inventions described in U.S. Pat. Nos. 3,992,267, issued Nov. 16, 1976; 3,824,167, issued July 16, 1974; 3,776,832, issued Dec. 4, 1973 and 3,909,386, issued Sept. 30, 1975. Each of these patents is incorporated by reference in its entirety herein. In addition, this invention is related to the invention disclosed in co-pending application Ser. No. 170,368 filed simultaneously herewith.
In the past, most electrochemical gas sensors utilizing a liquid electrolyte could only be used when maintained in one stabilized position. The sensor had to be oriented so as to make sure that all of the electrodes were in continuous contact with the electrolyte. As will be appreciated, the sensors of the past, subject to these limitations, were not properly operable in conditions where large amplitude vibrations might be present. Such vibrations, as will be understood, can momentarily isolate one or more of the electrodes from the electrolyte causing surges in the sensor response. Attempts to overcome these problems include utilizing a packing filling the entire content of the electrolyte cavity. While such a system provides a continuous contact between the electrodes, it reduces the quantity of electrolyte which may be introduced into a sensor of any given size, and because of the high absorbent qualities of the packing, it tends to disperse the electrolyte more or less uniformly throughout the cavity or container for the electrolyte. In the case of an aqueous electrolyte, such dispersed electrolyte will tend to degrade sensor performance as the sensor loses water by evaporation to dry sampled gas. Also, a small shrinkage in the volume of the packing will cause it to pull away from one or more of the electrodes and cause the sensor to cease functioning.
By contrast, the present invention through the use of the wick system, effectively provides appropriate contact on a continuous basis between the electrodes without excessively dispersing the electrolyte. Moreover, the electrochemical sensor, incorporating the invention herein is independent of its attitude and is not affected by environmental conditions such as excessive vibration or movement during the functioning thereof.
In sensors for use in determining the presence of hydrazine, potassium hydroxide may be used as the electrolyte. It has been determined, in accordance with this invention, that polypropylene is useful as the wick material in this environment. That is, polypropylene will not degrade in the alkaline environment of the electrolyte. Because of the general hydrophobic character of polypropylene, it will not act effectively as a wick material. It has been discovered, however, in accordance with a further aspect of this invention that polypropylene grafted with hydrophilic groups will act in a highly effective manner as a wicking material.